Polymeric adhesive paste

ABSTRACT

Adhesive paste of organic polymer resin, inorganic filler and fugitive liquid can be used for die attach applications.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation-In-Part of application Ser. No.08/594,345 filed Jan. 30, 1996, which was a Continuation-In-Part ofapplication Ser. No. 08/390,257 filed Feb. 17, 1995, and now U.S. Pat.No. 5,488,082, which was a Continuation-In-Part of application Ser. No.08/100,052 filed Jul. 30, 1993, and now U.S. Pat. No. 5,391,604.

BACKGROUND OF THE INVENTION

The present invention relates to an adhesive paste particularly wellsuited for bonding high density, microcircuit electronic components tosubstrates.

The attachment of high density, microcircuit components onto substrates,such as silicon dies onto ceramic sheet, has been an important aspect ofthe electronics industry for many years. Generally, it is known to use adie attach paste which is deposited between the die and substrate.Typically, the die attach paste includes a filler, an adhesive and acarrier. The filler is selected to impart to the finished bonding layerdesired conductive, resistive or dielectric properties. The adhesive ischosen to create a strong bond between the die and substrate. Thecarrier maintains all the components in a fluid, uniform mixture, whichallows the paste to be applied easily to the die-substrate interface. Italso has suitable volatility to migrate from between the die andsubstrate and leave a void-free bonding line following heat treatment ofthe assembly. After the paste is deposited and the die and substrate areassembled, the assembly is typically heated to fuse the adhesive anddrive off the carrier. Upon cooling, the die is firmly attached to thesubstrate.

For the well known silver-glass pastes with organic carrier, theassembly is typically heated to as high as 450° C. U.S. Pat. No.4,933,030 discloses a silver-glass paste for attaching high density,integrated circuits at temperatures as low as 300° C.

Silver-epoxy resin pastes based on liquid thermoset resins havepreviously been developed to permit die attachment at still lowertemperatures. However, epoxy resin pastes have certain drawbacks. Suchpastes achieve their adhesive and strength properties by crosslinking ofthe epoxy resin. The crosslinking process is generally time consumingand therefore uneconomical. Adjustment of the formulation to crosslinkfaster reduces the working time during which the paste can be applied tothe assembly, thereby complicating the process. Epoxy resins are twopart systems consisting of resin and hardener which can require separatestorage equipment and must be thoroughly mixed prior to use.Furthermore, epoxy resins generally crosslink to a rigid, unyieldingstate which can create excessive stress in especially large diesattached to high expansion substrate material. Thus, the die size thatcan be attached by an epoxy resin paste may be limited. Still further,as a crosslinked material, epoxy resins are not reworkable and thereforeshould not find appreciable use in the emerging, multi-chip modulemarket.

Thermoplastic polymer resin adhesives can be used for die attachment.However, such adhesives suffer the drawback that they are solid at lowtemperatures and thus must be pre-formed to appropriate shape for eachattachment application. Furthermore, solid adhesives are incompatiblewith the industry-standard, high speed production lines based on pasteadhesives. In addition, the use of solid adhesives requires applicationof pressure to bring about a satisfactory bond. Attempts have been madeto provide fluid adhesives based on thermoplastic polymer resinsdissolved in solvents. Unfortunately, these systems also suffer fromdeficiencies, most notably, low upper limits on the amount of silverthat can be incorporated while maintaining good adhesive and rheologicalproperties, and the tendency of voids to form following solventevolution unless excessively long devolatilization times are used.

SUMMARY OF THE INVENTION

The present invention provides die attach pastes which can be processedat low temperatures; are strong, yet sufficiently elastic to bond largesilicon dies to more expandable substrates without inducing excessivestress; which produce a void-free bonding line; are reworkable asnecessary for the multi-chip market; and can be applied by equipment andprocesses in the industry without major modifications.

Specifically the present invention provides an adhesive paste consistingessentially of:

(A) about 5-50 volume percent organic polymer resin;

(B) up to about 50 volume percent inorganic filler; and

(C) about 30-70 volume percent fugitive liquid;

wherein each of the resin and the filler is present in particulate formwith a particle size up to about 110 microns, and wherein the liquid andorganic polymer are each substantially insoluble in the other.

In a preferred embodiment of the invention, the organic polymer resincomprises a blend of at least one thermoplastic polymer and at least onesecond polymer, the second polymer being present in an amount of atleast about 5% of the blend.

DETAILED DESCRIPTION OF THE INVENTION

Each of the principal components of the adhesive pastes of the presentinvention performs the function of the corresponding component ofconventional silver-glass pastes, namely, the organic polymer resinprovides adhesion, the inorganic filler, if present, provides electricalor thermal conductivity properties, or both, and the liquid provides avehicle for dispensing the paste between the die and substrate. Thispaste can be used in existing microcircuit assembly process equipmentbecause it has the same rheological and flow characteristics asconventional pastes. It requires heat treatment to much lowertemperatures than glass-based adhesives owing to the lower meltingpoints of thermoplastic polymers employed. Because the organic polymerresin is present in fine particulate form and the liquid is chosen sothat resin and solvent are substantially mutually insoluble, the liquidcan be volatilized during heat treatment, leaving a void-free bondingline. Furthermore, because the thermoplastic resins can be repeatedlymelted and solidified, those embodiments of the invention are reworkableand suitable for multi-chip module technology.

As used herein, the expression "consists essentially of" means that thecomposition may include additional components other than the principal,named components, provided that the additional components are notpresent in an amount sufficient to detract from the operability of thepresent invention.

Organic Polymer Resin

Polymer resins which can be used in this invention include any organicpolymer which a) is a solid at temperatures from about room temperatureup to the ambient temperature at which microcircuit electroniccomponents operate and b) softens, upon heating to a temperature abovethe temperature at which microcircuit electronic components operate, tobecome sufficiently fluid to create an adhesive bond between thecomponents. Thermoset resins can be used. Thermoplastic resins arepreferred, which can be repeatedly fused and solidified by heating andcooling to the appropriate temperature range. Combinations ofthermoplastic and thermoset resins can also be used, as will be morefully described below.

The temperature at which a thermoplastic polymer softens and becomesfluid is typically characterized by the Vicat Softening Point. The VicatSoftening Point for many common thermoplastic polymers can be obtainedfrom references well known to those skilled in the art or from theliterature provided by the polymer suppliers. Thus the practitioner ofordinary skill can readily check the suitability of a thermoplasticpolymer for use in a specific die attach paste by assuring that theVicat Softening Point lies both sufficiently above the circuitry servicetemperature to avoid softening during normal operation of the electroniccomponent, and sufficiently below the desired heat treatmenttemperature.

Representative thermoplastic polymers which can be used include, forexample, poly (phenylene sulfides), poly (ether sulfones), polyamides,polyesters, polycarbonates, polysulfones, polyacetals, polyvinylhalides, polyolefins, halogenated polyolefins, acrylic polymers, vinylpolymers and thermoplastic epoxy resins. The thermoplastic resin may bea resin which is capable of being thermoset but which is used attemperatures and conditions which will not thermoset the resin. Suchresins include phenol-formaldehyde condensates, urea ormelamine-formaldehyde condensates, casein, and gelatin, for example.Additional representative thermoplastic polymers which can be usedinclude copolymers, organic or inorganic substituted polymers, andblends of two or more thermoplastic polymers.

Representative polyamides include poly (hexamethylene adipamide), poly(epsilon-caprolactam), poly (hexamethylene phthalamide andisophthalamide).

Representative polyesters include poly (ethylene terephthalate) and poly(butylene terephthalate).

The resin can contain, in addition to the thermoplastic polymer, minoramounts of additives such as adhesion promoters, thermal stabilizers,antioxidants and tackifiers. Such additives are useful for extending theupper service temperature of the thermoplastic polymer and forincreasing the wetting of the surfaces of the die, substrate andinorganic filler materials by the thermoplastic polymer. Suchstabilizers, adhesion promoters, antioxidants and tackifiers are wellknown in the art. Particularly preferred tackifiers include thosecommercially available from Neville as Nevtac 99 and those commerciallyavailable from Hercules as RegalRez 1018, 1085 and 1094.

The polymer resin can comprise a blend of thermoplastic polymer and upto about 50% of at least one second polymer. The second polymer can beselected from thermosetting polymers, such as epoxy resin; provided thatthe thermoplastic nature of the blend is retained. For example, theorganic polymer resin can comprise a blend of at least one thermoplasticpolymer and at least one thermoset polymer. When the thermoset polymeris an epoxy resin, the epoxy resin should be present in the blend in anamount of about from 10 to 50 volume percent. Epoxy resin can be addedto the adhesive either in powder form or in liquid form as solutedissolved in the liquid.

Thermoset resins, including epoxy resins, can be used in the presentinvention, alone or in combination with each other and withthermoplastic resins. The thermoset resins used in the present inventionare in particulate form, and, as such, obviate many of the difficultiespreviously encountered with thermoset resins in liquid form, as notedabove. The particular resin or combination of resins should be selectedin view of the intended application or use. For example, some wirebonding operations in high volume lines are done at temperaturesexceeding 230° C. At this temperature, most thermoplastic resins becometoo soft and can cause "swimming" of the die during the ultrasonicexcitation of the wire bonding tool, resulting in poor strength andunreliable wire bonds. Mixtures of miscible thermoplastic and thermosetpolymers, referred to as interpenetrating polymer networks (IPN), canhelp raise the allowable wire bonding temperature, but 100% thermosetresins are most effective in this regard.

Reactive polyester resins are preferred thermoset resins. Epoxy resinsarc more hydrophyllic (absorb water) whereas polyester is morehydrophobic (resists water). In some applications, a hydrophyllic resincould cause reliability problems in the package. If the resin absorbstoo much moisture, the molded packages could pop open (popcorning)during a rapid higher temperature excursion such as reflow soldering.

Other thermosetting resins that can be used include polyimide, acrylic,and bismaleimide powders.

It is important that the organic polymer resin and any filler be presentin the paste in fine particulate form, occasionally referred tohereafter as "powder". Some thermoplastic resins are available from thevendor as powder. Others are typically supplied in sheet, pellet, orgranular form. Thermoplastic polymer resins not supplied as powderhaving the desired size distribution characteristics can be comminutedby various well known techniques, such as for example, hammer milling,pin milling, abrasive wheel milling and cryogenic grinding.

The maximum particle size of organic polymer resin suitable for use inthis invention is about 110 μm. If the maximum powder particle size islarger than about 110 μm, appearance of the heat-treated assembly willbe depreciated by surface voids in the fillets resulting from particlesdetaching from the adhesive surface. It has been found that largerparticle size also promotes the tendency to form voids in theheat-treated bonding line. While not wishing to be bound by a particulartheory, it is believed that the bonding line voids may be produced whenlarge thermoplastic polymer resin particles hold the die away from thesubstrate during heat treatment. Under such conditions, liquid vaporizesfrom the paste, causing shrinkage parallel to the bonding line tocompensate for the volume of the lost liquid. The minimum particle sizeof the resin is not critical and is limited by the economics ofcomminution.

Inorganic Filler

The inorganic filler is present in amounts of up to 50 volume %. Whenpresent, it imparts desirable thermal or electrical properties to thebonding line. Many metals or ceramics well known in the electronicsindustry can be used. Preferred inorganic fillers include, for example,silver, gold, copper, and nickel and alloys thereof, alumina, beryllia,silica, silicon carbide, graphite tungsten carbide, barium titanate,steatite, boron nitride, aluminum nitride and diamond. The inorganicfiller also should be present in fine particulate form and the maximumparticle size should be about 110 μm.

The filler is preferably a noble metal, and silver has been found to beparticularly satisfactory. Silver powder is typically supplied in flakeor spherical particle form. When supplied as flake, silver particles mayhave a lubricant, typically a fatty acid, such as stearic acid, on thesurface as a result of the flaking process. Such lubricants normallydecompose at 250° C. or lower and do not ordinarily detract from theoperation of this invention. However, the presence of lubricant mayaffect the selection of liquid, as will be explained hereinafter.Generally, silver particulates which can be used in this invention aresmaller than the thermoplastic polymer resin powder. Silvers which canbe used include those having a surface area in the range of about from0.2 to 3 m² /g and a tap density in the range of about from 2 to 5g/cm³.

It is also possible to add an amount of low melting point alloy incombination with the inorganic filler to reduce the particle-to-particleor particle-to-interface contact resistance. The low melting point alloycan be in the form of solder balls which are admixed with the inorganicfiller in the adhesive mixture. Upon heat treatment, the solder ballsmelt to bridge from one particle of the inorganic filler to another.

The electrically or thermally conductive inorganic filler can also beincorporated in the paste as a metal resinate, such as silver resinateor zinc resinate. In this way, the inorganic filler can be precompoundedwith a polymer resin or coated on polymer particles, such as spheres,prior to addition to the paste.

Liquid

The fugitive liquid in the present invention functions to suspend theother ingredients so that they can be conveniently dispensed and appliedto the die and substrate. Furthermore, the liquid diffuses from thepaste and vaporizes during heat treatment to provide a substantiallyliquid-free, treated adhesive.

The vapor pressure of the liquid should be sufficiently low that it doesnot rapidly evaporate from the paste at room temperature. This is toavoid reducing the "working life" of the paste. Additionally, if thevapor pressure is too high, it may vaporize during heat treatment toorapidly, which may produce a bond line containing excessive voids. Thevapor pressure should be high enough to completely vaporize from thepaste within a commercially practical time during heat treatment. Thevapor pressure will therefore, at least in part, depend on theconditions of heat treatment. Nevertheless, because the presentinvention is particularly well suited to low temperature die attachment,the liquid should have room temperature vapor pressure, preferably, ofless than about 50 mm Hg.

It is important that the liquid is eliminated during treatment in such away as to provide a bonding line that is substantially free of voids.Generally, low surface tension and nonpolar liquids provide void-freebonding lines, and are therefore preferred. Representative liquids whichcan be used are aliphatic and aromatic hydrocarbons, and glycol ethersand their derivatives such as glycol ether acetates, having a boilingpoint of about from 150° C. to 275° C. Particularly satisfactory arealiphatic hydrocarbons.

It is believed that reaction between the lubricant residue normally onthe surface of silver flake and the liquid may cause voids in the bondline. Thus it is desirable that the liquid used be compatible with theinorganic filler and resin in the paste to the extent that voids are notgenerated on removal of the solvent.

The liquid is preferably a nonsolvent for the polymer resin. That is,each of the resin and the liquid is not significantly soluble in theother. However, a slight solubility, up to about 20 percent of the totalresin, and preferably, less than 10 percent, can be tolerated. If theliquid is more soluble in the resin, liquid may take too long to diffuseout of the fusing resin to the surface of the bond area. If the resin ismore soluble in the liquid, it may tend to form a film barrier whichexcessively retards the devolatilization of liquid from the bondingline. As heat treatment progresses in such circumstances, the liquid inthe bond line vaporizes and expands to produce voids.

The use of a co-solvent system has been found to be effective. In thiscase, one of the solvents should preferably be a solvent for the resinand the second selected as a non-solvent for the resin. For example, thecombination of dissolving methyl benzoate and non-dissolving aliphatichydrocarbon has been found to be specially effective in the presentpastes. The partially dissolving co-solvent system dissolves a smallportion of the polymer resin, which increases the viscosity, improvesthe rheology and suspension power, and decreases the solvent bleed atthe edge of the wet deposit.

Generally, organic liquids known for their use in conventional,glass-filled die attachment paste, can be used in the present invention,provided that such organic liquids possess the volatility and solubilityproperties identified above. The liquid can be a solution of two or moreliquid compounds.

Small amounts of a viscosity modifying polymer can be dissolved in thefugitive liquid to increase liquid viscosity while maintaining thevolatility and solubility properties stated above. Increased liquidviscosity enhances paste rheology, i.e., thickens the paste, whichimproves control and versatility of paste dispensing onto the parts tobe bonded. Suitable viscosity modifying polymers are substantiallysoluble in the fugitive liquid. Many thermoplastic elastomers arebeneficial for this additive including styrene ethylene propylene blockcopolymers. Other representative viscosity modifying polymers include,for example, styrene-ethylene/butylene-styrene block copolymers ("SEBS")and butyl rubber. SEBS, such as KratonTm G polymers from Shell arepreferred. Kraton™ G1702X diblock styrene-ethylene-propylene has beenfound to be particularly satisfactory. Blends of such polymers can beused as well. The type and concentration of the viscosity modifyingpolymer in the liquid will affect the extent of viscosity enhancement.In general, the viscosity modifying polymer is present in the range ofabout from 0.5 to 5 volume %, preferably about from 1-3 volume %, andmost preferably about 1.25 volume % of the fugitive liquid. Preferably,the viscosity modifying polymer is predispersed in the liquid prior topreparing the paste.

High surface area, solid, non-dissolving powders can also be used tomodify the viscosity or rheology of the paste, as well as controllingthe bleeding of the solvent around the edge of the fillet. For example,high surface area Ag flake, 50-S, from Degussa and high surface areasilica from Degussa can be included in the solvent system, before addingthe solids. The uniform dispersion of the high surface area powderincreases the viscosity of the solvent system. In general, the Degussa200 Aerosil powder was most effective when used in combination with theKraton G1702X viscosity modifying polymer, both predispensed in theliquid prior to preparing the paste. Generally speaking, the volumepercent of the high surface area powder is less than about 2% tooptimize the functional properties of the adhesive paste.

Die Attach Adhesive and Use

A particular advantage of this die attach adhesive is its similarity inperformance to conventional, glass-based adhesive pastes, except for itsability to undergo heat treatment at relatively low temperatures.Therefore the preparation of the adhesive from its principal components,and its methods of application and use, take advantage of the variousmethods and employ equipment well known in the art. In effect, this dieattach adhesive is a low treatment temperature, "drop-in" replacementfor inorganic- (i.e., glass-) based die attach and thick film pastes.

The die attach adhesive of the present invention is a uniform mixture ofpolymer resin, inorganic filler and liquid. The principal components canbe mixed in equipment known in the art for paste preparation. It shouldbe recognized that thermoplastic polymers chosen for a specific pasteapplication will have significantly lower fusion point than the glass ofconventional pastes. Accordingly, it may be necessary to operate themixing apparatus at slower speed and/or less intense agitation; withslightly smaller recipe of ingredients; with cooling; or with acombination of the preceding, in order to avoid coalescing of thethermoplastic polymers. These techniques may be necessary to assure thatthe thermoplastic polymer resin is present in fine particulate formprior to heat treatment.

The order of mixing the ingredients is not critical. All the principalcomponents can be blended together. Alternatively, combinations of twoprincipal components can be premixed followed by addition of othercomponents to produce the paste. For example, it may be desired to firstproduce a dry blend of polymer resin powder and inorganic filler byknown methods of powder mixing, such as by using ribbon or double-conetumble blenders. Thereafter, liquid can be added to the dry mixture toform the paste.

The adhesive composition prior to heat treatment will preferably containabout 24-37 volume % inorganic filler. The compositions more preferablycontain about 11-37 volume % and especially, about 13-29 volume %organic polymer resin.

It is known that thermoplastic polymers typically have maximum servicetemperatures above which they degrade, increasingly decomposing as thecombination of high temperature and exposure time becomes more severe.The ordinary skilled practitioner will appreciate that heat treatmenttemperature cycles customary for glass pastes should be adjusted toavoid degradation of the organic polymer.

The liquid also should provide a smooth paste which dispenses cleanly tothe bond line without dripping or tailing. Normally, the viscosity ofthe paste can be adjusted by changing the proportions of ingredients, aswill be evident to the skilled practitioner.

The die attach adhesive of this invention is also useful in many thickfilm paste applications, such as for example, the fabrication of hybridcircuits in which conductive, resistive and dielectric pastes are screenprinted onto substrate materials and heat treated to bond the desiredprinted circuit to the substrate.

The die attach adhesive of the present invention is typically used forattaching microcircuit electronic components to a substrate. In general,this comprises making an adhesive paste of the present invention;followed by applying the paste to a surface of a substrate to form abond line and placing the electronic component on the bond line so thatthe paste is between the electronic component and the substrate;followed by heating the assembly to a sufficiently high temperature fora sufficient time that the organic resin softens and becomes fluid, butdoes not degrade, and the liquid devolatilizes from the paste; followedby cooling the heat-treated assembly to a temperature below which thethermoplastic polymer becomes solid, whereby the microcircuit electroniccomponent is bonded to the substrate by a void-free bond line. Whenthermoset resin is used as part or all of the organic polymer, theprocessing temperature should be sufficiently high to crosslink theresin.

In some cases, it is desirable to "B" stage an adhesive, or partiallycure it, to allow a component to be subsequently attached. Component asused herein can include, for example, an integrated circuit, integratedcircuit package, a flip chip, a ball grid array (BGA) package and a landgrid array (LGA) package. By applying heat and pressure to a componentafter "B" staging or drying, the binder (thermoplastic, thermoset, or acombination) remelts or crosslinks and makes a bond. When usingthermosets, the B stage heat treatment must be low enough to avoid anylarge degree of crosslinking.

This type of material and process could be used in many applications. Aparticularly useful application is in the attachment of flip chipdevices to a substrate. Typical flip chip devices have "bumps" on theconductive pad of the chip. These bumps are used to attach the chip,after it is "flipped" over, to conductive traces on a substrate. Thebumps have historically been solder and are simply reflowed to attach.More recently, as described in detail in U.S. Pat. Nos. 5,196,371 and5,237,130 of Epoxy Technology, Ag epoxy pastes have been substituted forthe solder bumps and then attached to the substrates by an epoxy basedpolymer printed on the substrate. The cured polymer bumps on the chipare aligned in the wet paste on the substrate and cured.

With the technology of this invention, "B" staging bumps on the chipan/or the substrate would offer significant advantages. In particular,if the bumps were printed on the substrate and "B" staged cured, thechip could be bounded by heat and pressure. The combination of thefugitive liquid system with the undissolved polymer resin results in athixotropic rheology ideal for fine detail printing required for thisapplication. Additionally, the use of a thermoplastic resin allows for areworkable, low stress attachment.

To illustrate the feasibility of this application, a material similar toExample 59, except with a somewhat higher resin to Ag ratio, was stencilprinted on a ceramic substrate yielding 4 mil (0.004") round bumps about0.003" in height. After drying, a chip was placed over the bumps on ahot plate at 175° C. with pressure applied. After cooling, the topsubstrate was removed with force indicating a bond was made between thetwo substrates via the bumps.

A similar application requiring "B" stageable material is the attachmentof ball grid array (BGA) or land grid array (LGA) packages. BGA packagesin the present art have an array of solder balls attached to theconductive pads on the bottom of the package, usually an FR4 epoxylaminate. The BGA packages are placed on the interconnecting substrateto align the solder balls with conductive pads on the substrate. Theconnection is made by reflowing the solder. This usually involvesfluxes. This approach has several drawbacks--cleaning the corrosivefluxes off after assembly, the work hardening and reliability issues ofsolder, and the higher stress on the larger BGA package. Using thetechnology of this invention to replace the solder balls has somesignificant advantages, among which are overcoming each of the drawbackslisted above for solder.

One requirement for a solder replacement paste is screenability. Thematerial should be capable of printing a wet deposit of at least about15 mils thick and about 15 mils wide on the LGA package. The idealrheology of this system allows this to happen. For example, a 15-20 milwet deposit was made by doctor blading a strip of adhesive paste similarto Example 56. The wet deposit was then dried at 100° C. for 30 minutes.Then, a 0.400 in² die was clamped with a 1# clip over the B stagedmaterial and the assembly then brought to 175° C. for 15 minutes.Excellent adhesion strengths of about 950 psi were achieved.

Another application for "B" staging is on lead frames used in highvolume integrated circuit (IC) manufacturing. Typically the paste isdeposited on the lead frame and the assembly then goes through a curingor firing process. By "B" staging the conductive adhesive on theleadframe, the whole operation of dispensing and curing can beeliminated. The lead frames can simply be heated to a temperaturesufficient to bond the die under pressure in a fraction of the timerequired for the paste process. This method of use of the invention notonly increases the throughput in the production line, but takes up muchless space, a significant advantage.

Many other applications of "B" staging exist in electronic and otherassemblies. The above were illustrated examples and does not mean toimply any limitation on the method of use of this invention.

EXAMPLES

This invention is now illustrated by examples of certain representativeembodiments thereof, where all parts, proportions, and percentages areby volume unless otherwise indicated. The examples are intended to beillustrative only, and modifications and equivalents of the inventionwill be evident to those skilled in the art.

The materials used in the examples are summarized in Table I.Thermoplastic polymer resins were supplied by the vendor in powder formor in sheet or pellet form. Those supplied as sheets or pellets wereground to powder form using a high rotation speed, abrasive wire wheel.All thermoplastic polymer resin powders were of a particle size lessthan 110 μm. All thermoset resins were used in the powder form assupplied by the vendor.

Examples 1-19 and Comparative Examples A-F

Filler was first combined with a portion of liquid to make a blend ofabout 37-44% filler. The blend was processed on a 3-roll mill touniformly disperse the materials and form a filler paste. The fillerpaste was combined with sufficient organic polymer resin and additionalliquid to produce a blend of composition as indicated in Table II. Thisblend was processed on a 3-roll mill to produce a substantially uniformconcentration, die attach paste.

To measure bond strength, 13 microliters of die attach paste wasdeposited on a bare ceramic sheet. An 8.6 mm×8.6 mm, silicon die wasplaced over the paste. The assembly was heated in a furnace at anaverage rate of 10° C. per minute until the temperature indicated inTable II was reached. The sample was maintained at temperature for theduration also indicated in the table. Thereafter, the sample was removedfrom the furnace and cooled to room temperature. An aluminum stud wasfastened to the die with epoxy adhesive and pulled off with a SebastianIII stud puller. Bond strength was recorded directly from the studpuller. To measure voids created in the bond line, a transparent glassmicroscope slide was substituted for the die and the procedure wasrepeated except that 70 microliters of paste was placed on the ceramicsheet. The glass slides used for voiding experiments were 18 mm×18 mm insize. Voids were characterized by visual inspection.

Results of the examples are also summarized in Table II. Table II islisted by volume percent to accommodate the densities of the variousfiller materials that could be used. Bond strength was considered goodif it exceeded about 4.5 kg to about 6.8 kg (10-15 lbs). Examples 1 and2 demonstrate that the die attach paste of the present invention canproduce strong, void-free bonds at heat treatment of short duration at290° C. and slightly longer at 350° C. In Comparative Examples A and B,the thermoplastic polymer resin of Examples 1 and 2 was diluted with 20%of polysulfone predissolved in methyl benzoate. Void formation wasmassive but reduced to a still unacceptable, but moderate amount, as thepercent of resin in solution decreased. A smaller amount of polysulfoneproduced void-free bond lines in Example 3-5, which also show decreasingbond strength with decreasing resin amount. In Example 6, 20%polysulfone produced a satisfactory bond line because the amount oftotal resin was in solution reduced.

Comparative Example C demonstrates the performance of epoxy as thepolymer resin. Bond strengths improve with increasing amounts of epoxy.However, strength is only marginal at over 30% epoxy in the paste.

Examples 9-11 show that the present invention can produce marginallystrong bond lines at less than 200° C. heat treatment.

Examples 12-19 demonstrate effectiveness of this invention at producingstrong, void-free bonds at treatment temperatures in the range of150-250° C. As temperature is lowered, treatment time is extended. Also,bond strength decreases with reduced thermoplastic polymer resincontent. Resistivity of Example 18 was measured as 57×10-6 ohm-cm, whichis higher than that of glass-based pastes but much lower than that oftypical epoxy-based pastes.

Comparative Example D shows that excessive solubility between liquid andresin produces massive voiding of the bond line. Massive voiding is seenin Comparative Examples E and F, which is believed to be due to theincompatibility of the solvent with the silver flake surfactant.

Examples 20-34

Using the general procedure of Examples 1-19, Examples 20-34 wereprepared using thermosetting polymer resins. A reactive polyester resin,such as 3X111 from Tomoegawas (Examples 32 and 33) is preferred becauseof the combination of low resistivity, high adhesion and minimum voids.

                  TABLE I                                                         ______________________________________                                        Thermoplastic                                                                 ______________________________________                                        Polymer Resin                                                                   TP1 UDEL P3703 (Amoco) Polysulfone                                            TP2 UDEL P1800 (Amoco) Polysulfone                                            TP3 Ultrason E (BASF) Poly(ether sulfone)                                     TP4 Fortron (Hoechst) Poly(phenylene sulfide)                                 TP5 MR11 (Phillips) Poly(phenylene sulfide)                                   TP6 V1 (Phillips) Poly(phenylene sulfide)                                     TP7 GR01 (Phillips) Poly(phenylene sulfide)                                   TP8 Siltem (General Electric) Poly(etherimide                                  siloxane)                                                                    TP9 5183 (Bostik) Polyester                                                   TP10 5157 (Bostik) Polyester                                                  TP11 4709 (Bostik) Polyester                                                  TP12 5164 (Bostik) Polyester                                                  Liquid                                                                        L1 EXXAL #9 (Exxon) isononyl alcohol                                          L2 NMP (Aldrich) methyl pyrrolidone                                           L3 Methyl benzoate                                                            L4 Terpineol 318 (Hercules) terpene alcohol                                   L5 673 (Exxon) Dearomatized aliphatic                                         L6 686 (Exxon) Dearomatized aliphatic                                         L7 Suresol 157 (Koch) aromatic diisopropyl                                     benzene                                                                      L8 DBE (Dupont) aliphatic dibasic ester                                       L9 Norpar 15 (Exxon) aliphatic hydrocarbon                                    Inorganic filler                                                              F1 15ED #001 (Metz) Silver flake                                              F2 15ED #173 (Metz) Silver flake                                              F3 3000-1 (Metz) Silver powder                                                F4 EA295 (Chemet) Silver flake                                                F5 Boron nitride                                                              F6 Graphite                                                                   F7 Aluminum Oxide                                                             F8 Copper                                                                     F9 Nickel                                                                     F10 Aluminum nitride                                                          F11 Silver/copper alloy                                                       F12 Metz #80 Silver flake                                                     F13 Metz #67 Silver flake                                                     F14 Chemet XLA 0103 Silver Flake                                              Thermoset Polymer Resin                                                       TS1 Epoxy 7P3000-G                                                            TS2 Epoxy LE3000-G                                                            TS3 Epoxy DK8-BLUE                                                            TS4 Epoxy Farboset 9146                                                       TS5 Ferro Acrylic 158C121                                                     TS6 Ferro Epoxy 152C200                                                       TS7 Ferro Polyester 156C105                                                   TS8 Ferro Polyester 156C102                                                   TS9 Ferro Polyester VP-188                                                    TS10 Farboil Epoxy Farboset 9025                                              TS11 Farboil Epoxy Farboset 9251                                              TS12 Tomoegawa Polyester 3X111                                                TS13 Tomoegawa Epoxy PO3CW24N                                               ______________________________________                                    

                                      TABLE II                                    __________________________________________________________________________    Polymer Resin     Liquid   Filler   Heat Treatment                                                                            Voids in                                                                           Bond Resistivity         Example                                                                            Type     Vol %                                                                             Type Vol %                                                                             Type Vol %                                                                             Time (min)                                                                          Temp (° C.)                                                                  Bond Line                                                                          Adhesion                                                                           μohm-m           __________________________________________________________________________     1   TP5      21.8                                                                              L3   54.8                                                                              F4   23.4                                                                              2     290   None 11.8 --                    A 80% TP5/20% TP2 21.8 L3 54.8 F4 23.4 2 290 High 51.8 --                     B 80% TP5/20% TP2 19.2 L3 60.2 F4 20.6 5 350 Moderate 52.7 --                  2 TP5 19.2 50% L8/ 60.2 F4 20.6 5 350 None 35.5 --                              50% L4                                                                      3 90% TP5/10% TP2 19.2 L3 60.2 F4 20.6 5 350 None 39.1 --                     4 90% TP5/10% TP2 17.2 L3 59.2 F4 23.6 5 350 None 37.7 --                     5 90% TP5/10% TP2 14.0 L3 59.8 F4 26.2 5 350 None 15.0 --                     6 80% TP5/20% TP2 13.4 L3 61.5 F4 25.1 5 350 None 24.1 --                    C TS4 12.3 L5 52.5 F4 35.2 5 305 None 4.1 --                                   7 TS4 22.2 L5 47.4 F4 30.4 5 305 None 5.5 --                                  8 TS4 30.4 L5 43.1 F4 26.5 5 305 None 7.3 --                                  9 TP9 17.5 L5 49.8 F4 32.7 8 175 None <4.5 --                                10 TP9 21.7 L5 48.5 F4 29.8 8 175 None 6.4 --                                 11 TP9 25.4 L5 47.4 F4 27.2 8 175 None 8.6 --                                 12 TP9 25.4 L7 47.4 F4 27.2 20 150 None 12.7 --                               13 TP9 25.4 L5 47.4 F4 27.2 10 250 None 12.3 --                               14 TP9 25.4 L5 47.4 F4 27.2 2 200 None 31.8 --                                15 TP9 25.4 L5 47.4 F4 27.2 24 175 None 25.9 --                               15 TP9 25.4 L5 47.4 F2 27.2 25 175 None 25.9 --                               17 TP9 21.3 L5 49.6 F2 29.1 25 175 None 29.5 --                               18 TP9 16.7 L5 52.1 F2 31.2 25 175 None 20.5 57.0                             19 TP9 11.7 L5 54.8 F2 33.5 25 175 None 7.3 --                                D TP9 15.9 L3 62.3 F2 21.8 25 175 High <4.5 --                                E TP2 16.0 L1 66.8 F1 17.2 5 350 High 8.6 --                                  F TP3 16.0 L1 66.8 F1 17.2 5 350 High 18.2 --                                 20 TS1 14 L9 53 F14/F12* 33 25 175 None 15.3 43.6                             21 TS1 22 L9 49 F14/F12* 29 25 175 None 35.0 57.5                             22 TS2 14 L9 53 F14/F12 33 25 175 Mass -- 52.6                                23 TS3 14 L9 53 F14/F12 33 25 175 Slight 40.8 27.5                            24 TS4 14 L9 53 F14/F12 33 25 175 Med -- 43.9                                 25 TS5 14 L9 53 F14/F12 33 25 175 None -- 31.0                                26 TS6 14 L9 53 F14/F12 33 25 175 Mass -- 41.0                                27 TS7 14 L9 53 F14/F12 33 25 175 None -- 123.9                               28 TS8 14 L9 53 F14/F12 33 25 175 None -- 80.1                                29 TS9 14 L9 53 F14/F12 33 25 175 None -- 103.5                               30 TS10 14 L9 53 F14/F12 33 25 175 Mass -- 40.0                               31 TS11 14 L9 53 F14/F12 33 25 175 Med -- 94.3                                32 TS12 14 L9 53 F14/F12 33 25 175 None 48.0 23.2                             33 TS12 22 L9 49 F14/F12 29 25 175 None 62.5 23.9                             34 TS13 22 L9 49 F14/F12 29 25 175 None 38.3 93.1                           __________________________________________________________________________     *1:1 ratio                                                               

Examples 35-44

Adhesive pastes were prepared as in Examples 1-19 from the materialslisted in Table I in proportions shown in Table III. The ratio of fillerto organic polymer resin was 87:13 parts by volume. In each of theseexamples, the filler was a 50/50 blend, by weight, of F12 and F13, and42 volume % of liquid L9 were used. In all cases, silver resinate(Engelhard #9144) was added to the liquid in the amount of 3% by weight.

The pastes were tested for chip resistor adhesion as follows. Holes of1.6 mm (1/16 inch) diameter were drilled between each set of tinned padson a printed circuit board adapted to receive 1206 chip resistors. Theboard and pads were washed with acetone and the pads were abradedslightly. Three layers of 0.064 mm (2.5 mil) thick pressure sensitiveadhesive tape was placed on the board over the region where the chipresistors would be attached. The tape over the attachment sites was cutwith a knife and removed. The cut out sites were arranged in a daisychain pattern to permit a continuous electrical circuit through multipleresistors for contact resistance testing. Adhesive paste was applied tothe boards with a doctor blade, then the tape was removed to leave apaste thickness of 0.14-0.15 mm (5.5-6.0 mils). Type 1206 chip resistorswere manually placed on the board overlying the holes and with onlyterminations contacting the paste. The boards were fired at 175° C. fordwell time of 8 minutes, unless otherwise noted in Table III.

The fired boards were set, resistors facing down, on ceramic spacers anda 1.6 mm (1/16 inch) diameter extender rod connected to an Ametek forcegauge was inserted in a predrilled hole. Using a drill press to forcethe rod against the resistor, the adhesion force at bond failure wasdetermined as shown in Table III.

Example 45

Adhesive paste was prepared as in Examples 1-19 with no filler andwherein the organic polymer resin was entirely TP9.

After firing at 150° C. with a dwell time of 12 minutes, the pasteproduced an adhesive strength of 2.72 kg in the 1206 chip resistoradhesion test as in Examples 35-41.

The paste was also tested as a sealing material for a ceramic lid. 50 μLof the paste was dispensed uniformly around the gold plated seal ring ofeach of standard ceramic side braze packages which were then fired at150° C. for 15 minutes to remove volatile components. A ceramic lid wasassembled on each package covering the once fired, seal ring. Whileclamped under 0.68 kg(1.5 lbs) pressure, the assemblies were again firedat 150° C. for 15 minutes to produce a sealed cavity within the package.Packages passed standard military specification hermetic seal testing at-65, 25 and 125° C., i.e., they were found to have low leak rates.

Examples 46-49 and Comparative Examples G TO I

Adhesive pastes were prepared as in Examples 1-19, except that amountsof viscosity modifying polymers, as shown in Table IV, were dissolved inthe fugitive liquid prior to mixing the paste.

                  TABLE IV                                                        ______________________________________                                                                   Vol. % Polymer in                                    Example Viscosity Modifying Polymer fugitive liquid                         ______________________________________                                        46      SEP (Kraton ™ G1702X)                                                                         1.25                                                 47 SEBS (Kraton ™ G1654X) 3.00                                             G SEBS (Kraton ™ G1657) 5.00                                               48 SEP (Kraton ™ G1701X) 2.50                                              H SEBS (Kraton ™ G1765X) 5.00                                              I Butyl Rubber 5.00                                                           49 Exxon Vistalon ™ 3708 3.50                                            ______________________________________                                    

The compositions of Examples 46-49 provided satisfactory to excellentadhesive performance.

                  TABLE III                                                       ______________________________________                                        ORGANIC POLYMER RESIN                                                                                        Thermoplastic                                       Resin:                                                                        Thermoset                                                                  Example Thermoplastic Thermoset Resin Adhesion                                No. Polymer Resin Polymer Resin Parts/vol (kg)                              ______________________________________                                        35     TP9        None       100:0    1.68                                      36 None TS1 0:100 1.46 *                                                      37 None TS2 0:100 1.67                                                        38 None TS3 0.100 1.08 *                                                      39 TP9 TS3 50:50 2.22                                                         40 TP9 TS2 50:50 1.60                                                         41 TP9 TS1 50:50 2.30                                                         42 TP9 TS1 65:35 2.13                                                         43 TP9 TS1 80:20 1.82                                                         44 TP9 TS1 90:10 2.53                                                       ______________________________________                                         * Fired at 150° C., dwell time 12 minutes                         

Example 50

The general procedure of Example 1 was repeated except that athermoplastic powder was used having a particle size distribution from 0to 105 microns. Only a small percentage of the material was 100 micronparticles and the mean particle size was also much less than 100 micron,but the maximum particle was, at least 100 microns.

    ______________________________________                                        Paste Formulation:                                                                Material           Source   % age                                         ______________________________________                                        #67 Ag Flake       Degussa   78.75%                                             5183 0-105 μm TP Powder Bostik  8.75%                                      Norpar 15 Solvent Exxon  11.88%                                               Kraton G1702X TP Rubber Shell  0.25%                                          Silver Resinate Englehard  0.37%                                              Totals  100.00%                                                             ______________________________________                                    

The test material was dispensed with a hand dispenser onto a bareceramic substrate in an "X" pattern. 7.5 microliters was used for the0.250×0.250" square die. The die was pressed into the material until aminimum bondline was reached (would not go down further). Theceramic/die/material assembly was then placed in a 175° C. oven andfired at a standard processing profile. Temperature was recorded with athermocouple attached to a similar die/ceramic setup. Time totemperature was 71 minutes (25-175° C.) and time above 175° C. was 15minutes. The peak temperature was 177° C. The parts were pulled from theoven and sheared off approximately 1 hour later. Die shearing was donewith a modified Sebastian III stud puller. Maximum shear adhesion wasrecorded.

TEST RESULTS

0-105 micron Test Material #F369--18-21# on 4 parts, 20# AverageDM4030SD (-325 Mesh Material)--Typically 35-50# Average

The 100 micron TP powder exhibits sufficient adhesion and functionalitysuch that it could be usable as a satisfactory die attach medium.

Examples 51-52

Adhesive pastes were prepared as in Examples 1-19 according to thecomposition of Example 59, to which solder balls were added as shown inTable IV, in place of the silver flake. The solder balls were PbSneutectic composition and about 45 microaverage diameter. Firing at 200°C. for 10 minutes produced results as shown in Table IV, indicating thatthe addition of solder balls improves the resistivity of the resultingbond.

                  TABLE IV                                                        ______________________________________                                        Example Description        Adh.   ρ                                                                              Voids                                  ______________________________________                                        51      With 1.6% solder ball inclusion                                                                  49.5   55.0 Slight                                   52 With 3.2% solder ball inclusion 43.0 55.0 None                           ______________________________________                                    

Examples 53-58

To test the process of partially curing (B staging) the adhesive paste,Examples 53-58 were prepared using the general procedure of Examples1-19, with components as noted in Table V. The solvent system used wassimilar to that of Examples 59-62. Each of these examples, which usethermoplastic and thermoset resins and blends, thereof, exhibitedacceptable results for adhesion and resistivity.

                                      TABLE V                                     __________________________________________________________________________                                   "B" stage                                                                             Component Attach                       Resin              Filler      Temp                                                                              Time                                                                              Temp                                                                              Time                                                                              Pressure                                                                           Adhesion                                                                           ρ                Example                                                                            Type  Ratio                                                                            Vol %                                                                              Type                                                                              Ratio                                                                             Vol %                                                                             (° C.)                                                                     (mins)                                                                            (° C.)                                                                     (mins)                                                                            (psi)                                                                              (#)  (μohm-m)          __________________________________________________________________________    53   TP11/TS12                                                                           6:4                                                                              34.7 F13/F12                                                                           1:1 15.7                                                                              100 60  175 15  6    148  66.3                   54 TS12 n/a 36.1 F13/F12 1:1 16.3 100 60 175 15 6 397 56.3                    55 TP9 n/a 37.8 F13/F12 1:1 16.9 100 60 175 36 6 95 70.3                      56 TP11/TS12 6:4 33.7 F14 n/a 15.3 100 60 175 15 6 111 80.7                   57 TP11/TS12 6:4 36.1 F13 n/a 16.3 100 60 175 36 6 145 76.7                   58 TP11/TS12 6:4 36.1 F13/F3 9:1 16.3 100 60 175 36 6 175 76.4              __________________________________________________________________________

Examples 59-62

Adhesive pastes were prepared and tested as in Examples 1-34 accordingto the following compositions:

    ______________________________________                                        Resin            Filler        Liquid*                                                              Vol             Vol       Vol                             Example Type Ratio % Type Ratio % Type %                                    ______________________________________                                        59     TP9     N/A    25.4 F12/F13                                                                             1:1  26.2 L9   48.4                            60 TP11/ 6:4 25.4 F12/F13 1:1 26.2 L9 48.4                                     TS7                                                                          61 TP12/ 3:7 25.4 F12/F13 1:1 26.2 L9 48.4                                     TS12                                                                         62 TS12 N/A 25.4 F14/F12 1:1 26.2 L9 48.4                                   ______________________________________                                         *2.5% by volume SEP (Kraton G1702X) was predissolved in L9.              

Excellent adhesive performance was obtained using these pastes, using aprocess similar to that used in Examples 1-19.

We claim:
 1. An adhesive paste consisting essentially of:(A) about 5-50volume percent organic polymer resin; (B) up to about 50 volume percentinorganic filler; and (C) about 30-70 volume percent fugitiveliquid;wherein each of the resin and the filler is present inparticulate form with a particle size up to about 110 microns, andwherein the liquid and organic polymer resin are each substantiallyinsoluble in the other.
 2. An adhesive paste of claim 1 wherein theorganic polymer resin consists essentially of thermoplastic polymer. 3.An adhesive paste of claim 1 wherein the organic polymer resin consistsessentially of at least one thermosetting polymer.
 4. An adhesive ofclaim 3 wherein the organic polymer resin is a reactive polyester.
 5. Anadhesive paste of claim 1 wherein the organic polymer resin comprises ablend of at least one thermoplastic polymer and at least one secondpolymer, the second polymer being present in an amount of at least about5% of the blend.
 6. An adhesive paste of claim 5 wherein the secondpolymer comprises at least one thermosetting polymer.
 7. An adhesivepaste of claim 6 wherein the second polymer consists essentially of areactive polyester resin.
 8. An adhesive paste of claim 7 wherein thereactive polyester comprises at least 10 volume percent of the blend. 9.An adhesive paste of claim 6 wherein the second polymer consistsessentially of epoxy resin.
 10. An adhesive paste of claim 9 wherein theepoxy resin comprises at least about 10 volume percent of the blend. 11.An adhesive paste of claim 1 wherein the fugitive liquid comprises aneffective amount of at least one viscosity modifying polymer.
 12. Anadhesive paste of claim 11 wherein the viscosity modifying polymercomprises about from 0.5 to 5 volume % of the fugitive liquid.
 13. Anadhesive paste of claim 11 wherein the viscosity modifying polymerconsists essentially of thermoplastic elastomer.
 14. An adhesive pasteof claim 13 wherein the viscosity modifying polymer consists essentiallyof styrene ethylene propylene block copolymer.
 15. An adhesive paste ofclaim 1 wherein the fugitive liquid comprises an effective amount of ahigh surface area solid non dissolving powder.
 16. An adhesive paste ofclaim 15 wherein the high surface area powder comprises about from 0.5to 5 volume percent of the fugitive liquid.
 17. An adhesive paste ofclaim 16 wherein the high surface area powder consists of at least oneof silver powder having a surface area of at least about 4 m² /gm andsilica powder having a surface area of at least about 10 m² /gm.
 18. Anadhesive paste of claim 1 wherein the fugitive liquid further comprisesa solvent that dissolves a portion of the organic polymer resin.
 19. Anadhesive paste of claim 18 wherein the solvent comprises up to about 20%of the fugitive liquid.
 20. An adhesive paste of claim 1 furthercomprising about from 0.5 to 5 volume %, based on the fugitive liquid,of metal resinate.
 21. An adhesive paste of claim 20 wherein the metalresinate consists essentially of silver resinate.
 22. An adhesive pasteof claim 1 wherein the inorganic filler comprises silver particles andsolder balls.
 23. An adhesive paste of claim 22 wherein the volumetricratio of silver to solder is about from 10 to 1 to 40 to
 1. 24. Aprocess for assembling an electronic component on a substrate,comprising the steps of;i) depositing an adhesive paste on a substrate,the paste consisting essentially of;(A) about 5-50 volume percentorganic polymer resin; (B) up to about 50 volume percent inorganicfiller; and (C) about 30-70 volume percent fugitive liquid;wherein eachof the resin and filler is present in particulate form with a maximumparticle size up to about 110 microns, and wherein the liquid andorganic polymer resin are each substantially insoluble in the other. ii)placing an electronic component on the substrate in contact with theadhesive paste; iii) heating the resulting assembly to a temperatureabove which the polymer resin softens and becomes fluid and below atemperature at which the thermoplastic polymer resin begins to degrade,and; iv) cooling the assembly whereby the thermoplastic polymer resinsolidifies to bond the electronic component to the substrate.
 25. Aprocess comprising the steps of:a) depositing an adhesive pasteaccording to claim 1 on an electronic component or a substrate, b)partially curing the adhesive on the component or substrate; and c)bonding the component of step (b) to a substrate with heat and pressure.26. An article made by the process of claim
 24. 27. An article made bythe process of claim 25.